Elastic Support of Power Plant Equipment

Elastic support of Fans

The most common fans in power plants are ID-, FD- and PA-fans of axial as well as centrifugal type. Their imbalances caused by wear and tear during operation result in periodical dynamic forces, which are fully transmitted into the foundation and beyond. Therefore, fan foundation design must consider not only static but also these dynamic loads resulting ultimately in a foundation concept which is practical as well as acceptable in both the technical and economical way. Spring support of fans can conveniently meet these requirements. World-wide acceptance of this concept as a state of the art technology is of course our best evidence.

Conventional fan foundations at power plants typically require very massive RCC-blocks but this huge mass alone cannot stop the transmission of vibration into the soil/surrounding at all. It is mainly meant to suppress the bad effects of unintentional system resonances, which cannot be ruled out in this case, with very little knowledge of the exact dynamic properties of the soil.
Furthermore there is also the risk of settlements, especially uneven settlements due to these vibrations. It is, therefore, common practice to support the conventional foundation at several meters below ground level where the soil bearing capacity is much more than adequate for just the static loads. Such a conventional foundation should therefore be replaced by a better alternative.

GERB recommends dynamic uncoupling of the fan foundation from the soil by means of spring units equipped with high quality, fatigue free coil springs and non-ageing Viscodampers®. This system provides the following advantages:

  • Instead of a massive RCC-block, the fan foundation is reduced to a RCC-slab of very limited thickness and the dynamic behaviour of this system can be exactly calculated.
  • System natural frequencies are far away from the speed of the fan ensuring a very smooth running of the fan itself.
  • The fan foundation can be dynamically fully uncoupled from the sub foundation and soil. The vibration isolation efficiency is very high and may reach more than 90%.
  • The substructure below the spring system can be designed for the dead load of the machine and its RCC-deck only. It may be reduced to either walls or frames
  • which will conveniently rest on the soil without settlement problems. Piles, wherever required for a conventional foundation, can be greatly reduced in numbers or even fully eliminated.
  • The highly flexible spring support combined with Viscodampers® will even result in lower stresses in the machine itself i.e. an increase in bearing life, longer maintenance free periods and higher machine availability.

This foundation concept represents, therefore, an economic and timesaving solution providing besides good vibration isolation an excellent protection against settlements and even against earthquakes. The danger of uneven settlements is greatly reduced due to following reasons:

  • The soil is only statically loaded, while most settlements after commissioning are caused by dynamic loads
  • Up to a certain extent uneven settlements are automatically levelled out due to the flexibility of the springs.
  • If settlements should in spite of all this reach critical dimensions, they can easily be corrected by shimming on spring level in a few hours.